Abstract
The use of structured/random packing in vacuum column design has become an acceptable option, not only for revamps but also for new column designs. In addition, the development of new high-efficiency steam ejector designs, environmental concerns, and the high cost of energy have had considerable impact on the economics of the wet versus dry decision.
For a dry column to obtain the same degree of vaporization as a wet column, it has to operate at a substantially lower flash zone pressure. To pull a deeper vacuum requires more ejector motive steam. However, a dry column does not require velocity steam, stripping steam or precondensers, and produces a substantially smaller volume of effluent sour water than a wet system. In a wet system, the vacuum level is limited by the precondenser heat sink temperature.
This paper discusses the pros and cons of wet vs. dry vacuum column designs in view of current distillation technology and economic climate. An in-depth analysis is presented based on an actual case study from the recent design of a world-class grassroots refinery. The column design approach, available options for internals, vacuum system, pressure drop versus energy tradeoffs, heat recovery issues, utilities cost and overall optimization are discussed in detail.